Along with the recently increased demand for portable electronic products such as laptop computers, video cameras, mobile phones, and so on, the development of electric vehicles, energy storage batteries, robots, satellites, and so on started in earnest and led into active researches on high-performance secondary batteries capable of repeated charging and discharging.
Currently, commercially available secondary batteries comprise nickel cadmium, nickel hydrogen, nickel zinc and lithium secondary batteries. Among them, lithium secondary batteries have drawn much attention because of little memory effect to allow unrestrained charging/discharging, as well as very low self-discharging rate and high energy density, compared to nickel-based secondary batteries.
Meanwhile, there are cases where such a secondary battery is used in a single battery, but in many cases, the secondary battery is used in a state where a plurality of the secondary batteries are connected in series and/or in parallel so as to be used in a high voltage and/or mass energy storage apparatus. Further, the secondary battery is generally used in the form of a battery pack or a battery system that includes a plurality of battery cells and a battery management apparatus configured to control overall charge/discharge operations of the battery cells.
Further, such a battery management apparatus used in the battery pack performs the function of monitoring a state of the battery using a temperature sensor, a current sensor, a voltage sensor and the like and using results of such monitoring to estimate a state of charge (SOC) or a state of health (SOH) or to balance a voltage between the battery cells, or to protect the battery from high voltage, overcurrent, low temperature and high temperature, etc.
Meanwhile, in many cases, a conventional battery management apparatus measures and uses an open circuit voltage (OCV) of the battery cell in order to perform the management functions such as estimating the state of charge (SOC), estimating the state of health (SOH) and cell balancing, etc. The open circuit voltage of a battery cell refers to the voltage of the battery cell in a state where an external load or an external circuit is not connected or in a stabilized state where a charge/discharge operation of the battery cell is not performed. Here, the reason why the open circuit voltage of the battery cell is different from the actual voltage measured from the battery cell is known to be because of internal resistance (ohmic polarization) of the cell and the polarization phenomenon and the like related to movements of electrons in an electrode/electrolyte interface.
But, in order to measure an exact open circuit voltage, it is necessary to stop using the battery cell, and sufficient time must lapse prior to making the measurement. That is, in order to measure the exact open circuit voltage, the battery cell needs to be in a non-charged/discharged state.
However, in many cases, information on an open circuit voltage is required in real time in the process of charging/discharging. Further, depending on the type of apparatus where the battery is being applied, there are cases where charging/discharging continues without any non-operation period of the battery. For example, an energy storage system (ESS) used in frequency regulation of power generation equipment performs charging/discharging continuously without a rest time. Therefore, in the case of a battery used in such an apparatus, there is a problem that it is difficult to measure the open circuit voltage.
(Prior art document) Korean Laid-open Patent no. 10-2009-0020470 (laid-open on Feb. 26, 2009)